An electron microscopic histochemical assay for cyclic GMP phosphodiesterase has been developed for study of the retinal photoreceptor cells in the mouse. It is proposed to examine the developing retina of normal mice and mice wth hereditary retinal degeneration with this assay to determine if any abnormalities of phosphodiesterase are present in the mutant animals. It is further proposed to use the technique developed in mouse retina to study rod and cone photoreceptors in the human eye to learn if differences exist in the pattern of enzyme distribution in these two kinds of photoreceptor cells. Because of the small amount of tissue involved in the dystrophic mouse retina, and in view of the difficulty of separating rods and cones in the human retina, an electron histochemical technique is particularly well suited for the proposed investigations.